1. Field of the Invention
This invention relates to a method of track accessing in an optical recording apparatus and, in particular, to a novel technique for a beam spot to quickly cross the plural tracks while the number of tracks the beam spot has crossed is accurately checked.
2. Description of Related Art
An optical recording disk system is hereinafter described as an example of prior art optical recording systems. In an optical recording disk system, as shown in FIG. 1A, data is written onto a recording track on a recording medium, a disk 1, by a beam spot focused thereon and the data is read out from a reflected light therefrom. The system employed in FIG. 1A is constituted as follows. The optical recording disk 1 is rotating around its axle driven by a motor 1a. An optical head 2 coarsely travels along a radial direction of the disk which is driven by a motor. The motor which is not shown is controlled by a carriage controller 5. The optical head 2 is constituted so that a light emitted from a semiconductor laser 24, as a light source, is introduced via a lens 25 and a polaroid beam splitter 23 into an object lens 20. The light is focused by the object lens 20 as a beam spot BS so as to project onto the optical disk 1, and a light reflected from the optical disk 1 is input via the object lens 20 by the polaroid beam splitter 23 into a 4-division light receiver 26. The optical head 2 may be referred to as a carriage 2.
In such an optical recording disk, tracks are formed with a pitch of several micrometers, typically 1.6 .mu.m, in great numbers along a radial direction of the optical disk 1. Accordingly, even a little eccentricity, typically 100 .mu.m peak to peak, may cause a large track-displacement for a narrow single track, and an undulation of the optical disk 1 causes a displacement of a focusing point of the beam spot as well. Therefore, it is necessary for the beam spot to be less than 1 .mu.m to follow these displaced and/or undulated tracks.
For these purposes, there are provided a focus-actuator (focus coil) 22 for moving the object lens 20 on the optical head 2 along the axial direction of the lens so as to change the location of the focusing point, and a track actuator (tracking coil) 21 for moving the object lens 20 (i.e. along left and right directions in the figure) so as to change the location of the beam spot along the orthogonal (i.e. radial) direction of the tracks. There are also provided a focus servo controller 4 for generating a focusing error signal FES from the light signal of the light receiver 26 to drive the focus actuator 22, and a fine tracking servo controller 3 for generating a tracking error signal TES from the light signal of the light receiver 26 to drive the track actuator 21.
The principle of fine tracking control depends, as shown in FIGS. 1B and 1C, on the utilization of a diffraction phenomena of the beam spot BS caused by a coaxial or spiral guide groove (track) 10 provided in advance on the optical disk 1. In other words, the fact that distribution of the amount of the reflected light into the light receiver 26 is varied by the light diffraction at the track 10 depending on the relative position of the beam spot BS to the track 10, is utilized in order to acquire the beam spot's position error from the track 10. For example, when a push-pull method having a four-division light receiver which consists of four photo diodes 26a, 26b, 26c and 26d is employed as it is conventionally used, the reflected light distributions at the light receiver 26 are such that: for the case where the location of the beam spot P.sub.1 is deviating toward left hand side of the track 10 as shown in FIG. 1C, the distribution is as shown in FIG. ID; for the case where the beam spot P.sub.1 is not deviating from the track 10 (i.e. on track), the distribution is as shown in FIG. 1E; and for the case where the location of the beam spot P.sub.2 is deviating toward right hand side of the figure from the track 10, the distribution is as shown in FIG. 1F. Therefore, {(a+b)-(c+d)} obtained in the fine tracking servo controller 3 from the outputs a-d of the photo diodes 26a-26d forms a tracking error signal TES shown in FIG. 1G, which then drives the tracking actuator 21, which accordingly drives the object lens 20 toward left and right directions. Thus, the beam spot can be controlled to trace the track 10 regardless of the eccentricity and undulation of the optical disk 1.
For the beam spot to cross many of the tracks to access a particular track (referred to hereinafter as track jumping), two basic methods have been employed. The first one of the methods is to move the beam spot by moving the optical head (a carriage) 2, on which there are installed the lens 20 or a galvano mirror (not shown in the figure) thereon, along the orthogonal directions of the tracks for a long stroke, such as 120 mm. The second method is to move the beam spot with a fine tracking mechanism, which is originally provided for the beam spot to trace a track by moving the lens 20 or the mirror (representatively referred to hereinafter as lens) on a carriage along orthogonal directions of the tracks in order to compensate an eccentric movement of the track, while the carriage is fixed or following the lens by a lens position servo control (details of which will be described later), for the case where the number of track jumping is less than one hundred, for example. These two methods are used generally in combination. The number of tracks the beam spot is crossing is acquired by counting zero cross transitions in a zero cross signal TZCS which is generated from a tracking error signal which is originally provided for the fine tracking servo control of the lens.
As previously mentioned, the optical disk, and therefore each track, rotates undesirably eccentrically as much as 100 .mu.m peak to peak, typically having a frequency of 30 Hz for 1800 revolutions per minute. The amount 100 .mu.m corresponds to about 60 tracks having a pitch of 1.6 .mu.m. Accordingly, in the first method an erroneous zero cross signal is generated when the radial speed of an eccentric track exceeds the beam spot's radial speed, resulting in an error in the counting of the number of track crossings. To determine the location of the beam spot after generation of an error in the counting of the number of track crossings, an ID (identification) number recorded in each track has to be read out after the track jumping is finished, and then according to the read out track ID number, the beam spot location must be adjusted by moving the lens to reach the destination track. Consequently, track jumping takes a long time.
In the second method, the counting of the crossed tracks is correct if the track crossing is done track by track. In this track-by-track jump method, a track actuator 21 drives the object lens 20 to move the beam spot BS from a present track 10a to a destination track 10d as shown in FIGS. 2A and 2B. A single-track jumping is repeated until the destination track is reached. Each track jumping involves a sequence in which an acceleration current i.sub.a is applied to the track actuator for an acceleration and next a deceleration current -i.sub.a is applied for a deceleration to stop. In each single track jumping, the beam spot BS can not trace the track quickly and stably after the single track jump is carried out because a settling period is required to allow the beam spot to capture the track through adjustments provided by the fine servo (the beam spot is considered to have captured the track when tracking error signal TES becomes almost zero). It takes a relatively long period, such as 2.5 ms, for the beam spot to cross to an adjacent track and to stop at the centre of the track. Accordingly, the period required for jumping a great number of tracks becomes the multiplication of the 2.5 ms by number of tracks to be jumped. Such a long period can not satisfy the recent trend of fast accessing of the optical disk.